1. Field of the Invention
This invention relates a projection lens which allows production of a small size projection-type display apparatus, a projection-type display apparatus using the projection lens, and a projection-type display apparatus capable of changing the contrast and the luminance.
2. Description of Related Art
FIG. 1 is a diagram showing an optical system of a conventional projection-type display apparatus using liquid crystal panels, which apparatus is disclosed, for example, in Japanese Patent Application Laid-Open No. 1-120192. In this figure, indicated at 1 is a light source including a lamp 120 and a reflecting mirror 130. The light source 1 emits an illumination beam 2. As a lamp 120 is used, for example, a white light source such as a metal halide lamp, a xenon lamp, and a halogen lamp. A reflecting surface of the reflecting mirror 130 is typically a paraboloid or spherical surface, and a luminescence center of the lamp 120 is positioned at a focus position of the paraboloid or a center of curvature of the spherical surface as well known. Along an emitting direction of the illumination beam 2 are arranged a dichroic mirror 14B which reflects a blue light while allowing green and red lights to transmit therethrough, a dichroic mirror 14G which reflects the green light while allowing the red light to transmit therethrough, and a mirror 11a for reflecting the light entirely. A mirror 11c for reflecting the light entirely is arranged in a reflecting direction of the dichroic mirror 14B, and a liquid crystal panel 3B is arranged in a reflecting direction of the mirror 11c. A liquid crystal panel 3G is arranged in a reflecting direction of the dichroic mirror 14G. A mirror 11b for reflecting the light entirely is arranged in a reflecting direction of the mirror 11a, and a liquid crystal panel 3R is arranged in a reflecting direction of the mirror 11b. A drive circuit for causing an image to be displayed on the liquid crystal panels 3R, 3G, 3B is unillustrated. A dichroic prism 15 which selectively reflects the red and blue lights and selectively allows the green light to transmit therethrough is arranged and surrounded by these liquid crystal panels 3R, 3G, 3B. A projection lens 4 is arranged in an emitting direction of the dichroic prism 15. The respective members constituting the optical system before the projection lens 4 are held in a casing 200, and the casing 200 and the projection lens 4 constitute a projector 300.
An operation of the projector 300 will be described next. The illumination beam 2 emitted from the light source 1 is separated into lights of three primary colors, namely red, green, and blue, by the dichroic mirrors 14B and 14G, and the respective color lights are reflected by the mirrors 11a, 11b, 11c and projected on the liquid crystal panels 3R, 3G, 3B for displaying monochromatic images corresponding to the respective primary colors. The lights modulated by the images formed on the liquid crystal panels 3R, 3G, 3B are combined into a single beam again by the dichroic prism 15. The combined single beam is converted into a projected light 110 by the projection lens 4 to be projected on a screen as described later. The projection lens 4 is required to correct various aberrations in order to obtain a satisfactory projected image, and includes a combination of a plurality of single lenses (not shown). The projected image on the screen is focused by driving a part of lens elements constituting the projection lens 4 or the entire projection lens 4 along an optical axis.
FIG. 2 is a diagram showing an optical system of another conventional projection-type display apparatus using the liquid crystal panels, which apparatus is disclosed, for example, in Japanese Patent Application Laid-Open No. 1-157688. In this figure, indicated at 131 is a condenser lens provided at the front portion of a light source 1. In this example, the light source 1 includes a lamp 120, a reflecting mirror 130, and the condenser lens 131. Other construction is same as the aforementioned prior art (see FIG. 1), and no description will be given thereon by giving like reference numerals to like parts. Further, since the operation of this display apparatus is similar to the one of the aforementioned prior art, description thereof is omitted.
As a projection-type display apparatus, there have been conventionally known a projection-type display apparatus of the front system by which the projected light emitted from the projection lens is formed as an image on a reflection type screen, and a projection-type display apparatus of the rear system in which a projector is accommodated in a cabinet and the projected light is formed as an image on a translucent screen. FIG. 3 shows a construction of a projection-type display apparatus of the rear system disclosed in Japanese Utility Model Application Laid-Open No. 1-115778. In this figure, indicated at 170 is a cabinet for accommodating a projector 300 including a casing 200 and a projection lens 4. The projector 300 has a configuration similar to that of the projector 300 in FIG. 1 or 2. In the casing 200 are held an optical system (not shown) including from a light source 1 to a dichroic prism 15. On a side wall of the cabinet 170 is arranged a translucent screen 5. In the cabinet 170 are arranged bent mirrors 150 and 160 for introducing a projected light 110 emitted from the projection lens 4 to the screen 5.
An operation of the apparatus shown in FIG. 3 will be described next. The projected light 110 emitted from the projection lens 4 is formed as an enlarged image on the screen 5 after being reflected by the bent mirrors 150 and 160. The bent mirrors 150 and 160 are employed to bend an optical path extending from a leading end of the projection lens 4 to the screen 5 so as to accommodate the optical system in the cabinet 170 in a compact manner. A viewer 400 views the enlarged image from a direction opposite to the projector 300 relative to the screen 5.
In the projection-type display apparatus of the rear system, a portion 171 below the screen 5 is large in the cabinet 170 since the bent mirror 150 and the projector 300 are accommodated below the screen 5 as shown in FIG. 3. In view of this, it has been difficult to reduce the height H of the optical projection set. Thus, there has been adopted a method of projecting the projected light 110 directly behind the screen by omitting the bent mirrors 150 and 160 from the optical system so as to set the height H substantially equal to the level of the screen 5. In this case, the problem is such that the depth D of the set is increased to thereby increase an installation area of the cabinet 170.
In addition to the above, it has been effective to shorten a projection distance (length of a central beam from an emitting end of the projection lens 4 to the screen 5) in order to make the set small size. Accordingly, there has been adopted a method of widening an angular field of the projection lens 4. However, if a divergent angle of the projected light 110 is widened by widening the angular field of the projection lens 4, the large size bent mirror 150 is inevitably required. Thus, there has been a limit in reducing the height H of the set.
Conventionally, there has been a demand to view an image of the increased luminance since the projected image becomes relatively dark and hard to see when the luminance on the screen surface is high due to the external light. Conversely, when the luminance of the external light is low, there has been a demand to view an image by improving the contrast (particularly lowering the luminance of a black level) while suppressing the luminance of the projected image. However, the conventional projection-type display apparatuses are not provided with a function of automatically adjusting the luminance and the contrast according to the illuminated state of the place where the apparatus is installed.